Sealed impeller for producing metal foam and system and method therefor

ABSTRACT

A system for producing a metal foam comprises a bath containing a molten metal, a rotating shaft or impeller extending through the base of the bath into, and submerged in the molten metal, and a gas discharge nozzle provided on the submerged end of the shaft. The opposite end of the shaft is connected to a gas supply line and the shaft is rotated with a motor. A seal is provided at the opening in the base of the bath for preventing leakage of the molten metal there-through.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to submerged impellers and, moreparticularly, to impellers used in generating metal foam.

2. Description of the Prior Art

There is a considerable demand for materials having high strength andlow weight characteristics for use in manufacturing various articles.Such materials are very much in demand in the automobile andconstruction industries. To meet this demand, metal foam has beenproposed. Metal foam is generally formed by introducing a gas into amolten metal bath to generate a foam on the surface thereof. Due to itshigh strength to weight ratio, aluminum is a favoured metal to use ingenerating a foam, although other metals can also be used. The foam isthen removed and formed or cast into the desired shapes. Various methodshave been proposed for introducing the gas into the molten metal bath.Such methods include the use of gas generating additives, blowing of airetc. With regard to the latter method, various apparatus and systems areknown for blowing a gas into the molten metal. Such apparatus includenozzles, impellers and other such devices.

In U.S. Pat. No. 5,334,236, there is described a metal foam generatingsystem wherein air is introduced by means of a gas nozzle at the end ofa supply tube or a hollow rotating impeller having a plurality ofopenings through which the gas is passed. In both cases, the tube orimpeller is mounted on an angle into the metal bath through an opening.There is no teaching in this patent as to how such opening is sealed toprevent the molten metal from leaking. Further, the shafts used informing the tubes or impellers are formed from stainless steel due tothe fact that they are immersed in molten metal. Nevertheless, suchshafts are known to become deteriorated after prolonged immersion in themolten metal and must be replaced often. Another deficiency in theseknown gas introduction systems is that since the shafts are provided inan angled manner into the molten metal bath, the length of the shaftsmust be adjusted if the depth of the bath is increased. Apart from thedrive mechanism requirements of such an arrangement, it will beunderstood that the cost for each shaft would also be greater. This,compounded with the need for constant replacement of the shafts, resultsin a light cost of operation.

In U.S. application No. 60/312,757, sharing a common inventor with thepresent application, an improved metal foam generating and castingsystem is provided. In this system, a metal foam is generated byintroducing a gas into the bottom of the metal bath to generate bubbles.The bubbles are then allowed to rise through a riser tube connected to adie cavity. The bubbles then form a foam inside the cavity. After thecavity is filled, it is allowed to cool and the formed metal foamarticle is retrieved. In this case, the generation of bubbles at aspecific location is desired. This reference provides a porous nozzlelocated at the bottom of the molten metal bath, positioned generallydirectly under the riser tube. Although such porous nozzle results inthe desired foam generation, a rotating nozzle is believed to improvethe foam characteristics. However, the rotating nozzle shafts known inthe art have various disadvantages as described above. In this specificapplication, one other disadvantage is that, with angled impellershafts, it is often not possible to ensure that the formed bubbles areintroduced into the riser tube. Further, the above mentioned systeminvolves the pressurization of the foaming chamber. In such case anadequate seal around the impeller is needed in order to prevent leakage.Such seal is difficult to establish in situations where the impeller isintroduced through the side of the molten metal bath.

Thus, there exists a need for an improved impeller system for generatingmetal foam.

SUMMARY OF THE INVENTION

Thus, in one embodiment, the present invention provides a submerged gasdischarge impeller for supplying a gas to liquid within a container,said impeller comprising:

-   -   a hollow shaft having at least one bore and a first end        connected to a gas supply and a second end extending into said        liquid through an opening in the bottom of said container;    -   the second end of said shaft including a gas discharge nozzle in        fluid communication with said bore;    -   the shaft including a seal for preventing leakage of said fluid;    -   a drive means for rotating the shaft about its longitudinal        axis.

In another embodiment, the invention provides a system for discharging agas through a liquid, the system comprising:

-   -   a container for said liquid, said container having a base with        an opening;    -   a hollow shaft having a first end connected to a gas supply and        a second end extending into said liquid through said opening in        said container;    -   a gas discharge nozzle connected to said second end of said        shaft;    -   a seal provided adjacent said opening in said container for        preventing leakage of said liquid;    -   a motor connected to said shaft for rotating said shaft about        its longitudinal axis.

In yet another embodiment, the invention provides a system for producinga metal foam from a molten metal comprising:

-   -   a bath containing said molten metal, said bath comprising a        container with an opening on the base thereof;    -   a hollow, rotatable shaft extending generally vertically into        said molten metal through said opening, said shaft including a        first end extending into said molten metal and a second end        connected to a gas supply;    -   the first end of said shaft including a gas discharge nozzle        submerged in said molten metal;    -   a seal located between said shaft and said opening for        preventing passage of said molten metal;    -   a drive mechanism connected to said shaft for rotating said        shaft about its longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the inventionwill become more apparent in the following detailed description in whichreference is made to the appended drawings wherein:

FIG. 1 is a cross sectional elevation of a metal foam casting apparatus,

FIG. 2 is a cross sectional elevation of a detail of molten metal bathillustrating an impeller according to an embodiment of the presentinvention.

FIG. 3 is a side view of a gas supply mechanism for the impeller of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a metal foam casting system as taught in U.S.application No. 60/312,757, described above, in which the presentinvention can be used. As illustrated, the casting system includes a die36 having a die cavity 38, which is fluidly connected to a riser tube39. The riser tube 39 extends into a bath 32 containing a molten metal34. The bath 32 also includes, at the base thereof, a porous plug, ornozzle, 44. A gas supply line 42, connected to the nozzle 44, introducesa gas through the nozzle 44, into the molten metal 34. Such gas leads tothe formation of bubbles 46 which, due to their buoyancy, preferentiallyrise in the direction shown by the arrow C. As can be seen, bypositioning the riser tube 39 generally directly over the nozzle 44, thebubbles are caused to enter such tube and rise to form a metal foam. Aswill be appreciated the opening of the tube 39 may be provided with afunnel shaped end to assist in collecting the formed bubbles. The foamis, thereby, allowed to enter and fill the die cavity 38. As will beunderstood by persons skilled in the art, once the die cavity is filledwith the metal foam, the die can be cooled to solidify the foam and,subsequently, remove the formed foam article.

FIG. 2 illustrates a rotating gas supply impeller for use, in oneexample, as an alternative to the stationary porous nozzle of the metalfoam casting system described above and as illustrated in FIG. 1.

The rotating impeller according to one embodiment of the invention isshown generally at 100 in FIG. 2. The impeller includes a hollow shaft102 that extends generally vertically into the base 104 of the moltenmetal bath (not shown). As is commonly shown in the art, the bath,including the base 104, is provided with a refractory or insulatingmaterial 105 that is capable of withstanding the temperatures of themolten metal. A first, bottom end 106 of the shaft 102 provides andexposed opening 108 into the hollow bore 110 of the shaft 102. Air isintroduced into the bore 110 of the shaft 102 by connecting a gas supplyline (discussed further below) to the opening 108.

Turning briefly to FIG. 3, an example of a gas supply arrangement isillustrated. As shown, the shaft 102 includes a threaded portion (notshown) on the interior wall of the bore 110. A rotary union 160 includesa threaded connector 162 having a thread that is complementary to thatof the bore 110. The rotary union 160 is secured to the shaft 102 byscrewing the connector 162 into to the bore 110. The rotary union 160includes a rotating section 164 and a stationary section 166. The meansof linking sections 164 and 166 together is commonly known and, indeed,the rotary union 160 itself is commercially available. A gas supply port168 is provided on stationary section 166. A gas supply line 170 is thenattached to the supply port 168. Although preferred gas supply systemhas been described, various other methods of providing a gas supply tothe shaft 102 will be apparent to persons skilled in the art.

Returning to FIG. 2, on the second, top end 112 of the shaft 102, thereis attached a gas outlet nozzle 114. The top end 112 of the shaft 102extends into the molten metal bath through an opening 116, which extendsthrough the base 104 and refractory material 105. A support 118 having acentral bore 120 is provided in the opening 116 in the base 104. Theshaft 102 extends through the central bore 120 of the support 118, withthe central bore 120 being dimensioned to allow free rotation of theshaft 102. The support 118 includes a generally conical upper portion122, which includes an annular shoulder 124 that bears against a portionthe inner surface 126 of the base 104, such portion being adjacent tothe opening 116. The support 118 also includes a generally cylindricalbody 117, through which extends the bore 120, the body 117 preferablyextending through the opening 116. The outer diameter of the body 117 ispreferably dimensioned to provide a snug fit within the opening 116. Asindicated above, the upper portion 120 of the support 118 has agenerally conical structure. Such structure aids in directing moltenmetal away from the shaft 102. Although the support 118 and the opening116 are described in terms of preferred structural configurations, itwill be understood by persons skilled in the art that various othergeometries are possible within the scope of the present invention asdescribed herein. It will also be understood that the support 118 ispreferably made from a material that is capable of withstanding thetemperature of the molten metal. For example, suitable materials includealumina silicate, graphite or ceramics.

The central bore 120 of support 118 includes an upper region 121, at thetop end of the support 118, which has a larger diameter than that of thebore 120. Such widened diameter provides a ledge 128, which supports aseal or bushing 130. The bushing 130 has a generally cylindrical outerwall 132 that corresponds generally to the diameter of the upper region121 of the support 118. In the preferred embodiment, the bushing 130 ismaintained in position within the upper region 121 by frictional contactbetween its outer wall 132 and the inner wall of the upper region 121.Further, such arrangement ensures a tight seal between the bushing 130and the support 118. In the preferred embodiment, the bushing 130 ismade of graphite to withstand the temperatures of the molten metal towhich it is exposed. However, other materials will be apparent topersons skilled in the art such as ceramics, metals, or composites. Someexamples of possible materials for the bushing 130 include, inter alia,graphite, titanium diboride, tungsten, alumina, zirconium oxide (ZrO₂),silicon carbide, silicon nitrate, boron nitrate, titanium carbide andtungsten carbide.

In another embodiment, the support 118 can be integrally formed with theseal or bushing 130. However, it will be understood that a separate sealis preferred so as to facilitate replacement as the seal 130 wears out.It will also be understood that for forming an optimal seal, theunderside of the nozzle 114 should be square with the upper contactingsurface of the seal or bushing 130.

In a preferred embodiment, the material chosen for the seal or bushing130 is non-wetted by the molten metal. Similarly, the impeller or partsthereof is also made of a non-wetted material. In another embodiment,the elements in contact with the molten metal, i.e. the seal bushing130, the support 118, the nozzle 114, and any other parts of theimpeller, may be coated with a protective material that resists wettingby the molten metal and/or to seal the apparatus to prevent leakage.

The bushing 130 also includes a central bore 134, which accommodates theupper end of the shaft 102 and allow for rotation of the shaft therein.The clearance between the outer diameter of the shaft 102 and the bore134 of the bushing 130 is preferably maintained as minimal as possibleso as to provide a sealing arrangement there-between. In this manner,and with the seal between the bushing 130 and the support 118, leakageof molten metal within the bath is prevented.

The gas discharge nozzle 114 preferably comprises a generallycylindrical body secured to the top end of the shaft. In the preferredembodiment, the body of the nozzle 114 comprises a plurality of fins 115extending radially from the central axis of the body. The nozzle 114also includes a central opening 136 in fluid communication with thecentral bore 108 of the shaft 102. In the preferred embodiment, theopening 136 does not extend through the entire body of the nozzle 114and, instead, the body of the nozzle 114 is provided with one or more,and more preferably, a plurality of gas discharge vents 138 extendingthrough the fins 115. The vents 138 radiate from, and are in fluidcommunication with, the opening 136 of the nozzle 114. The vents 138open into the molten metal bath so as to discharge the gas suppliedthrough the shaft 102 into the molten metal. By securing the nozzle 114to the shaft 102, it will be understood that rotation of the shaft 102also results in the rotation of the nozzle. In the preferred embodiment,the bottom surface of the nozzle 114 abuts the top surface of thebushing 130 so as to establish a sealing arrangement there-between.

The shaft 102 extends through an opening in a stationary support 140located below the bath. The support 140 preferably includes a bearing142 having a central bore 144 that is greater in diameter than that ofthe shaft 102. The bore 144 is preferably provided with a bushing 146through which is passed the shaft 102. It will be understood that theshaft 102 is rotatably accommodated within the bushing 146. One of thepurposes of the bearing 142 is, as will be understood, to support andstabilize the shaft 102 while it is rotated. The bearing 142 ispreferably also provided with a washer 148 on the bottom thereof,through which is passed the shaft 102. The purpose of the washer 148 isdescribed below.

At the bottom end 106 of the shaft 102, there is provided a collar 150,secured to the shaft. Between the collar 150 and the washer 148, thereis provided a spring 152, the spring being in a compressed state. Aswill be understood, the spring, being provided in this manner, exerts aforce bearing against the washer 148 and the collar 150, causing thewasher and the collar to be forced away from each other. This force willextend along the length of the shaft 102 thereby causing the bottomsurface of the nozzle 114 to bear against the top surface of the bushing130, thereby serving to strengthen the seal between the nozzle and thebushing to prevent leakage of molten metal from the bath. It will alsobe understood that such force will also ensure that the support 118 ispressed against the inner surface of the bath to ensure a sealthere-between as well. It will be appreciated, however, that the primaryreason for applying a force by means of the spring 152 is to seal thenozzle against the bushing. Although the use of a spring 152 is apreferred method of achieving the desired seal, it will be understoodthat any other means may also be employed. For example, the shaft 102may be attached to any other force applying means to achieve the desiredresult. Alternatively, the weight of the shaft and associated elementsmay be sufficient to provide the necessary sealing force.

The present invention envisages various means of rotating the shaft 102.In one embodiment, the shaft 102 is provided with a pulley 154, securedto the shaft 102 in a location along the length thereof. The pulley 154translates a drive force applied thereto into axial rotation of theshaft 102. As is known in the art, the pulley 154 is adapted to engage adrive belt that is connected to a drive motor (not shown). In anotherembodiment, the pulley 154 may be replaced with a sprocket that engagesa cooperating sprocket on a drive shaft of a motor. The choice drivemeans for axially rotating the shaft 102 will depend upon the drivemechanism being used. It will also be understood that locating the drivemeans (for example the pulley 154) away from the bottom end 106 of theshaft 102 is preferred so as not to interfere with the gas supply linefeeding the bore 108.

In the preferred embodiment, a further bearing 156 is provided on theunderside of the base 104 of the bath. The bearing 156 can be, forexample, of the same structure as bearing 142 described above. It willbe understood that the purpose of the bearing 156 is to support andstabilize the shaft 102 while it is rotated. It will also be understoodthat in other embodiments of the invention, the bearing 156 may not beneeded if the shaft 102 is able to support itself. As shown, in thepreferred embodiment of the invention, the bearing 156 is also providedwith a bushing 157 similar to bushing 146. It will also be appreciatedthat any number of bearings or bushings can be used depending upon theneeds of the apparatus.

As described above, an impeller according to the present inventionimproves the dispersal of the gas discharged within the molten metal.Also, the impeller of the invention, by minimizing or eliminating thelength of the shaft exposed to the molten metal, avoids damage theretoas described above as well as other deleterious effects of having arotating shaft within the fluid molten metal. Also, by providing a meansof discharging gas directly from the bottom of the bath, the desiredvertical rise of the gas bubbles is achieved.

In the above described embodiments, a system having a single impellershaft and gas discharge nozzle has been described. However, theinvention also contemplates other systems wherein several impellers andnozzles are employed. As will be apparent to persons skilled in the art,more than one impeller and nozzle combination may be more efficient whenlarge diameter riser tubes 39 are used.

The present invention has been described in terms of its use in a metalfoam casting system. However, it will be appreciated that this is onlyone possible use of the invention and that various other uses are withinthe scope thereof. Although impeller speeds of around 4500 rpm are knownin art of metal foam generation, any other desired speed would, ofcourse, be possible.

Although the invention has been described with reference to certainspecific embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the spirit and scope ofthe invention as outlined in the claims appended hereto.

1. A submerged gas discharge impeller for supplying a gas to liquidwithin a container, said impeller comprising: a hollow shaft having atleast one bore and a first end connected to a gas supply and a secondend extending into said liquid through an opening in the bottom of saidcontainer; the second end of said shaft including a gas discharge nozzlein fluid communication with said bore; the shaft including a seal forpreventing leakage of said fluid; a drive means for rotating the shaftabout its longitudinal axis.
 2. The impeller of claim 1 wherein saidliquid is a molten metal.
 3. The impeller of claim 1 wherein saidimpeller is biased against said seal.
 4. A system for discharging a gasthrough a liquid, the system comprising: a container for said liquid,said container having a base with an opening; a hollow shaft having afirst end connected to a gas supply and a second end extending into saidliquid through said opening in said container; a gas discharge nozzleconnected to said second end of said shaft; a seal provided adjacentsaid opening in said container for preventing leakage of said liquid; amotor connected to said shaft for rotating said shaft about itslongitudinal axis.
 5. The system of claim 4 wherein said liquid is amolten metal.
 6. The system of claim 4 wherein said impeller is biasedagainst said seal.
 7. A system for producing a metal foam from a moltenmetal comprising: a bath containing said molten metal, said bathcomprising a container with an opening in the base thereof; a hollow,rotatable shaft extending generally vertically into said molten metalthrough said opening, said shaft including a first end extending intosaid molten metal and a second end connected to a gas supply; the firstend of said shaft including a gas discharge nozzle submerged in saidmolten metal; a seal located between said shaft and said opening forpreventing passage of said molten metal; a drive mechanism connected tosaid shaft for rotating said shaft about its longitudinal axis.
 8. Thesystem of claim 4 wherein said impeller is biased against said seal. 9.The system of claim 8 wherein said impeller is associated with a springfor biasing said impeller against said seal.
 10. The system of claim 7wherein portions of said system in contact with said molten metal areformed of a material that repels said molten metal.
 11. The system ofclaim 7 wherein portions of said system in contact with said moltenmetal are coated with a material that repels said molten metal.